1. Field of the Invention
The present invention relates to a radio-frequency module that separates a transmitting signal and a received signal.
2. Description of the Related Art
Hitherto, a radio-frequency module including a filter circuit has been provided. For example, a filter circuit 500 shown in FIG. 15 is mounted on a radio-frequency module (see, for example, Japanese Unexamined Patent Application Publication No. 2012-109818. The filter circuit 500 includes a filter unit 503 and a path 504. The filter unit 503 is connected between an input terminal 501 and an output terminal 502. The path 504 is connected in parallel with the filter unit 503. In this configuration, a signal path for allowing a RF (Radio Frequency) signal input into the input terminal 501 to pass therethrough is split into a path of the filter unit 503 and the path 504. When an RF signal is input into the input terminal 501, a first signal 505 passes through the filter unit 503, while a second signal 506 passes through the path 504. Then, the first signal 505 passing through the filter unit 503 and the second signal 506 passing through the path 504 are combined, and the resulting RF signal is output from the output terminal 502.
The filter unit 503 is defined by a band pass filter in which a predetermined pass band is set. The filter unit 503 allows an RF signal in the pass band to pass therethrough and attenuates an RF signal outside the pass band. However, the mere use of the filter unit 503 may not be sufficient to attenuate an RF signal outside the pass band to a desired value. Thus, a correction circuit defined by an inductor and a capacitor is provided in the path 504. The impedance of the path 504 is set so that the phase of the second signal 506 outside the pass band of the filter unit 503 which passes through the path 504 will be opposite in a frequency band outside the pass band of the filter unit 503 to the phase of the first signal 505 which passes through the filter unit 503 and which is desired to be attenuated further and so that the amplitude of the first signal 505 will be the same as that of the second signal 506 in a frequency band outside the pass band of the filter unit 503.
At a node between the signal line on the output side of the filter unit 503 and that of the path 504, the first signal 505 that has passed through the filter unit 503 and the second signal 506 that has passed through the path 504 cancel each other out in a frequency band outside the pass band of the filter unit 503, thereby attenuating an RF signal in this frequency band output from the output terminal 502. This can improve the attenuation characteristics for an RF signal outside the pass band of the filter circuit 500. Additionally, another filter circuit defined by a band pass filter, the pass band of which is different from that of the filter circuit 500, may be disposed adjacent to the filter circuit 500. In this case, it is possible to reduce the possibility that an RF signal outside the pass band of the filter circuit 500 output from the output terminal 502 will enter this adjacent filter circuit. This improves isolation characteristics between the filter circuit 500 and the adjacent filter circuit.
In order to improve the attenuation characteristics for an RF signal outside a predetermined pass band, the above-described known filter circuit 500 requires, in addition to the filter unit 503, the path 504 including a correction circuit to generate an RF signal of opposite phase to an RF signal outside the pass band passing through the filter unit 503. The provision of the path 504 increases the size of the filter circuit 500 and accordingly increases the size of a radio-frequency module including the filter circuit 500.